Process for fabricating silicon nanostructures

ABSTRACT

A process for etching a substrate comprising polycrystalline silicon to form silicon nanostructures includes depositing metal on top of the substrate and contacting the metallized substrate with an etchant aqueous solution comprising about 2 to about 49 weight percent HF and an oxidizing agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 121 as a division ofU.S. patent application Ser. No. 14/924,273, filed Oct. 27, 2015, titled“PROCESS FOR FABRICATING SILICON NANOSTRUCTURES,” which is acontinuation of U.S. patent application Ser. No. 14/444,361, filed Jul.28, 2014, titled “PROCESS FOR FABRICATING NANOWIRE ARRAYS,” now U.S.Pat. No. 9,202,868, which is a continuation of U.S. patent applicationSer. No. 13,305,649, filed Nov. 28, 2011, titled “NANOSTRUCTURED SILICONFOR BATTERY ANODES,” now U.S. Pat. No. 8,791,449, which is acontinuation of U.S. patent application Ser. No. 12/423,623, filed Apr.14, 2009, titled “PROCESS FOR FABRICATING NANOWIRE ARRAYS,” now U.S.Pat. No. 8,143,143, which claims priority to U.S. ProvisionalApplications Nos. 61/044,573, filed Apr. 14, 2008, and 61/141,082, filedDec. 29, 2008. These applications are incorporated by reference herein.

TECHNICAL FIELD

This application pertains to the field of nanotechnology.

BACKGROUND OF THE INVENTION

The ability to structure and pattern silicon is important for manyapplications. There has been particular interest in patterning siliconto make nanostructures. Relevant information regarding siliconfabrication processes known to those of skill in the art can be found,for example, in Sami Franssila, Introduction to Microfabrication (JohnWiley & Sons, 2004), and the references cited there.

Semiconductor nanowires have become the focal point of research over thelast decade due to their interesting physical, chemical and biologicalproperties. There is particular interest surrounding silicon nanowires,as silicon is one of the most abundant materials in the earth's crustand has become a cornerstone for many of the electronic, optoelectronic,electro-chemical and electro-mechanical devices upon which designs arebased.

Today, many nanosystems are not utilized commercially due to the largecost associated with fabrication, and limitations in the scalability ofnanowire synthesis. Nanowires have been grown bottom up using molecularbeam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), andphysical vapor deposition (PVD). They have also been fabricated top-downusing techniques like reactive ion etching (RIE) and inductively coupledplasma (ICP). These systems require high temperature and/or low pressurewhich is largely responsible for the high cost. A push towards solutionbased techniques that can be operated in ambient conditions is importantgiven their low cost, simplicity of design and ease of utilization.

Recent work has demonstrated the fabrication of silicon nanowires usinga solution made up of a metal salt and a strong acid (typically AgNO₃and HF). (See reference (a).) By controlling the concentrations of eachcomponent in solution, silicon can be etched normal to the plane of thewafer forming vertically aligned silicon nanowires with an averagediameter of 150 nm and a diameter range from 20-300 nm. Through therealization that silver is precipitating out of solution and catalyzingthe silicon etch, the technique has been modified to incorporate theaddition of H₂O₂ into the chemical bath and Ag metal directly depositedonto silicon. Polystyrene spheres of uniform dimensions were dispersedprior to the deposition of the Ag in order to use them as an etch maskand define the nanowire. (See reference (e).) As a result, orderedarrays of silicon nanowires with a homogeneous diameter and length weredemonstrated.

The ultimate diameter achieved with this technique has been limited. Theability to achieve sub-100 nm dimensions is of value to a variety ofelectronic, optoelectronic, electrochemical and electromechanicalapplications. For example, it is within the sub-100 nm range thatsilicon begins to demonstrate novel properties distinguishable from theproperties of bulk silicon. In addition, an increase in surface area atthe low nanometer scale is of value.

SUMMARY OF THE INVENTION

A process is provided for etching a silicon-containing substrate to formnanowire arrays. In this process, one deposits nanoparticles and a metalfilm onto the substrate in such a way that the metal is present andtouches silicon where etching is desired and is blocked from touchingsilicon or not present elsewhere. One submerges the metallized substrateinto an etchant aqueous solution comprising HF and an oxidizing agent.In this way arrays of nanowires with controlled diameter and length areproduced.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts the result of using an embodiment of the invention toobtain nanowires with diameters that ranged from 12-70 nm.

FIG. 2 depicts the result of using the alternative embodiment describedbelow in part B to obtain nanowires.

FIG. 3 depicts schematically a setup for practicing processes of theinvention, using oxygen as an oxidizer.

FIG. 4 depicts the result of using metal enhanced etching of silicon toobtain microstructuring on a silicon wafer. Unintentional wires wereformed inside the trenches.

DETAILED DESCRIPTION OF THE INVENTION

In an aspect of the invention, a process is provided for etching asilicon-containing substrate to form nanostructures. In this process,one deposits and patterns a metal film onto the substrate in such a waythat the metal is present and touches silicon where etching is desiredand is blocked from touching silicon or not present elsewhere. Onesubmerges the metallized substrate into an etchant aqueous solutioncomprising about 4 to about 49 weight percent HF and an oxidizing agent.

In a process as described above, to achieve sub-100 nm nanowires, onemay use sub-100 nm nanoparticles to block the silver from the silicon.The nanoparticles may be made of a variety of substances, for examplesilicon dioxide, iron oxide, or polymers.

A. First Exemplary Process

An exemplary process utilizes the spinning of SiO₂ nanoparticles welldispersed in an isopropanol solution (5 wt % SiO₂ in IPA) that ranged inparticle size from 12-30 nm at 4000 RPM. Subsequent to spin-coating, thesamples were heated at elevated temperatures resulting in solventevaporation. Once the samples were dry, 40 nm of Ag was sputterdeposited on top in order to coat the SiO₂ nanoparticles as well as thebare silicon spaces in between. The samples were dipped into an HF/H₂O₂solution for a period of 10 minutes. The etching reaction commenced atthe Ag/Si interface and the nanoparticles acted as a barrier by which tomask and define the nanostructure. The dimensions of the nanoparticles,which are selected per the desired application, influence the dimensionsand the shape of the resulting one-dimensional nanostructure. Someagglomeration occurred between the SiO₂ nanoparticles resulting in wiredimensions on the order of a single agglomerate. By selecting theconcentration of particles in solution as well as the method fordepositing/spin coating the particles, it is possible to limitagglomeration resulting in nanowires that ranged from 12-70 nm. A resultis depicted in FIG. 1.

B. First Alternative

In a variant on the process described above, one uses iron oxidenanoparticles (5-10 nm) with surfaces that have been pretreated witholeic acid and dispersed in chloroform. This surface treatment was donein order to prevent agglomeration and maintain a stable nanoparticlesuspension. In this situation it was not necessary to spin coat.Deposition of the nanoparticles onto the silicon substrate was achievedby applying a few drops of the solution to the surface under staticconditions (no spinning) Rapid evaporation at room temperature resultedin a monodispersed layer of the iron oxide particles on the siliconsubstrate with little to no agglomeration. Without wishing to be boundby theory, it is believed that the monodispersed layer resulted at leastin part from the pre-tailored surface tension properties and the highvapor pressure of the solvent.

In this alternative process, Ag was sputter deposited on the surface andthe substrate to coat the iron oxide particles and the spaces inbetween. The sample was dipped into a similar HF/H₂O₂ solution in orderto begin the etching reaction and form the nanowires. An example of theresult is seen in FIG. 2. There is some bundling that occurs due to thedrying process, making it difficult to determine precisely the actualnanowire dimension. However, all of the measurable structures had adiameter less than 30 nm.

C. Second Alternative

In a further variation of the process, other oxidizers may be used inplace of H₂O₂ in the H₂O₂-HF etchant solution. An oxidizing agent (alsocalled an oxidant or oxidizer) is a substance that readily transfersoxygen atoms or tends to gain electrons in a redox chemical reaction.One such oxidizer is pure oxygen, which may be introduced by bubblingoxygen through the HF. Other oxidizers include: ozone, chlorine, iodine,ammonium perchlorate, ammonium permanganate, barium peroxide, bromine,calcium chlorate, calcium hypochlorite, chlorine trifluoride, chromicacid, chromium trioxide (chromic anhydride), peroxides such as hydrogenperoxide, magnesium peroxide, dibenzoyl peroxide and sodium peroxide,dinitrogen trioxide, fluorine, perchloric acid, potassium bromate,potassium chlorate, potassium peroxide, propyl nitrate, sodium chlorate,sodium chlorite, and sodium perchlorate.

It may be desirable to use a less reactive alternative oxidizer in placeof H₂O₂. Comparative reactivity towards the metal deposited (e.g.,silver), silicon, or silicon dioxide may be of interest in the selectionof an oxidizer. Reactivity may be measured, for example, by the extentto which the reaction goes forward in a particular period of time, or bydetermining a reaction rate as discussed in books on physical chemistryand chemical kinetics. (See, e.g., Peter W. Atkins & Julio de Paula,Atkins' Physical Chemistry (8th ed. 2006), especially chapters 22 and23.) Measurements may be made in conditions such as temperature andpressure similar to those of the etching process.

An exemplary process would be as follows:

Silicon material with a resistivity of greater than 20 ohm-cm isselected having a surface with (100), (110), (111) or any orientationavailable. Amorphous and/or microcrystalline material will also resultin vertically oriented nanowires if one carries out the followingprocess.

The substrate is pre-cleaned using a series of solvents by sonicatingfor three minutes each in acetone, methanol and then isopropyl alcohol.The substrate is then rinsed in a dump-tank of flowing deionized water(DI) for 3 minutes to remove any residue remaining from the solventclean. The silicon is placed into a Piranha solution made up of 3 parts96% H₂S0 ₄ and 1 part 30 wt % H₂O₂ for 15 minutes in order to remove anyadditional organics and create a hydrophilic surface. The substrate isthen removed from the bath and placed into a dump dank of flowing DIwater again for 3 minutes to remove any residual acids. The substrate isremoved and blown dry with nitrogen gas.

A colloidal suspension of 10 nm iron oxide nanoparticles in chloroformis made by diluting product #SOR-10-0050 from OceanNanotech to aconcentration of 1 mg/mL. The silicon wafer is coated with the ironoxide by dipping the silicon into the colloidal suspension and thenremoving the substrate so that the surface normal is perpendicular tothe vertical direction of motion allowing the chloroform to sheet of thesurface. The combination of the hydrophilic surface and the nature ofthe oleic acid functionalized iron oxide nanoparticles, results in anatural self-assembly that limits agglomeration and gives somereasonable spacing to the particles. The samples are then baked on an80° C. hot plate for 2 minutes and cleaned using an in-situ O₂ plasmaprior to metal deposition.

In addition to iron oxide nanoparticles, 100 nm polystyrene spheres havealso been used successfully in this process. In this situation, ahydrophilic surface is created on the silicon substrate as describedabove. The polystyrene spheres (purchased from Duke ScientificCorporation) are diluted to a concentration of 1% and spun onto thesubstrate at 500 RPM for 5 seconds followed by a ramp to 2000 RPM for 40seconds. The polystyrene spheres create a single monolayer on thesurface. An O₂ plasma is used to reduce the size and spacing in thepolystyrene spheres inside a plasma stripper prior to inserting thesample into the metal deposition tool (30 W, 200 mTorr). The sampleheats up during the plasma clean, which can change the properties (ormelt) the polystyrene making it difficult to continue the shrinkingprocess. To resolve this, the polystyrene is etched using short 1 minuteintervals, removing the substrate from the tool and allowing it to coolto room temperature prior to the next 1 minute etch. Preferably theshrinking process is done in-situ (inside the metal deposition tool)prior to metal deposition with Ar or O₂.

Silver (Ag) is deposited via physical vapor deposition inside asputterer, thermal evaporator or e-beam evaporator. It is desirable thata continuous film results, where there are no breaks or cracks thatwould cause a portion of the film to become isolated from the remainder.As HF concentration is altered, the optimal film thickness may need tobe varied.

Once the chip is coated with the appropriate film of Ag, the HF solutionis seasoned before commencing the etching reaction. The concentration ofHF can vary from full strength (about 49 wt %) all the way down to verynominal concentrations. Initial observations have shown that the lengthof the resulting nanostructure increases as HF concentration is reduced.Concentrations as low as 2 wt % and below may be used. For example, asolution of 8 wt % HF may be used.

O₂ gas is flowed into the bath to create a vigorous bubbling for aperiod of 10 minutes. Once the bath is seasoned, the samples aresubmerged. At the completion of the etch, the samples are removed andput into a dump-tank of flowing DI water and blown dry with N₂. At thispoint the remaining Ag on the surface can be removed with a silveretchant, for example the etchant supplied by Transene Corporation.

FIG. 3 depicts a setup for the use of oxygen gas as an oxidizer. Thereis a container holding HF etchant 40. In the container there is asilver/silicon substrate 42. There is an oxygen source 44 which producesoxygen bubbles such as 46 and 48. The oxygen source inlet may be placedat, above, or below the level of the substrate. With this lessaggressive oxidizer, all wires were formed by the intentionalnanoparticle mask that was spun on top of the surface. No defects formedin the metal film during etching were apparent.

An advantage of the processes which use alternative oxidizers is thatthey are able to eliminate unintentional nanowires (“grass”) formed insome variants of the processes described above, while still producingthe desirable sub-100 nm nanowires while using a thin continuous layerof metal for catalyzed etching of silicon. FIG. 4 depicts unintentionalnanowires.

Avoidance of these unintentional nanowires both saves an etching stepwhich would be used to eliminate them and also avoids the rounding ofnanostructure corners and edges which would result from such an etchingstep.

While not wishing to be bound by theory, it is believed that some of thealternative oxidizers used in these processes of the invention do notattack the metal, or attack it to a much lesser degree than H₂O₂. Thismay be a reason why processes with alternative oxidizers avoid theformation of unintentional nanowires. For this reason it may bedesirable to use oxidizers known to react less readily or at a lowerrate with the metal than H₂O₂ does.

In order to avoid unintentional wires, it is desirable that the metalfilm be free of small unintentional holes and be deposited on a cleansilicon surface free of oxide.

A further advantage of the processes of the invention is that they areextendable to specified crystallographic directions. With at least someprocesses of the invention, irrespective of the crystal orientation ofthe silicon surface, the nanowires will be etched at least approximatelyperpendicular to that surface. In order to achieve this, it is desirablethat the metal film has no breaks or cracks that would cause a portionof the film to become isolated from the remainder. It is also desirablethat the metal film has sufficient adhesion and is deposited on a cleansilicon surface. Desirable angles between nanowire axes and a vectornormal to the substrate may be, for example, less than about 0.25degrees, about 0.5 degrees, about 1 degree, or about 2 degrees.

With processes of the invention, it is possible to make wires which havea noticeable taper that results in the wire diameter increasing slightlyas the etch progresses. The taper has been found to increase with HFconcentration. For some applications, the taper may not be desirable.For photovoltaic applications, however, a taper may be beneficial. Forexample, with a slight taper, free carriers in the nanowire will bounceoff the edges of the wire and thus tend propagate downward to thesubstrate. If the p-n junction of the photovoltaic cell is in thesubstrate rather than in the nanowires, this increased diffusion towardsthe substrate may be expected to increase the cell efficiency. Desirabletaper angles might be, for example, no more than about 0.5 degrees,about 1 degree, about 2 degrees, or about 4 degrees, or in a rangebetween about 0.5 degrees and about 1 degree, about 2 degrees, or about4 degrees.

Using processes of the invention it is possible to achieve nanowirediameters which are on average (e.g., have a mean or median) below about150 nm, below about 125 nm, below about 100 nm, below about 70 nm, orbelow about 50 nm. Small nanowires are important in certainapplications, such as where the small size changes the band structure ofthe silicon. It may be desired, for example, that a majority, or atleast about 75%, or about 90%, or about 95% of the nanowires havediameters less than a selected dimension such as those indicated above.

D. Applications

Processes of the invention may be applied to structuring silicon foroptoelectronic devices (see reference (i)). They may be employed indevices that utilize the photoelectric or photovoltaic effect, forexample solar cells (see, e.g., references (j) and (k)), photodetectors,photodiodes (see reference (a)), phototransistors, photomultipliers andintegrated optical circuits. Silicon nanowire arrays or individualnanowires fabricated via this process can be utilized within each ofthese applications.

Processes of the invention may be employed to produce devices made outof or comprising polycrystalline silicon. The invention encompassesprocesses which can be used with any crystalline orientation of silicon.Such processes can be used to texture the surface of and/or formnanowires in polysilicon. Polysilicon is a cheaper material thancrystalline silicon, but it is typically more difficult to texture andstructure than single crystal silicon due to the random orientation ofthe grains. The processes of the invention can likewise be used to formnanowires in amorphous silicon.

Arrays of silicon nanowires can be used in applications where thesilicon will be subjected to stress or strain where the nanostructure isable to absorb and relax this stress or strain. For example, nanowirescan act as an interfacial layer between bulk silicon and anothermaterial grown on top which is not lattice-matched to it.

Processes of the invention are also applicable to lithium ion batterytechnology. Silicon has been seen as a desirable candidate for the anodematerial in lithium ion batteries due to its low discharge potential andhigh charge capacity. Its application in the past has been limited dueto the large change in volume associated with ion insertion and ionextraction. The large amounts of stress and strain that builds in thesilicon results in degradation of the silicon layer resulting in a veryshort performance lifetime. Nanowires have been pursued due to theirability to withstand these stress and strains (see reference (1)). Theability to form well ordered and aligned nanostructures with a greatdeal of control over the resulting diameter and void spacing betweenthem provided by processes of the invention would be advantageous inmaking a lithium ion battery anode. In addition, the fact that poroussilicon (nanopores or micropores) can also be fabricated via processesof this invention would enable the fabrication of another anode geometrycapable of withstanding the stresses and strains of ioninsertion/extraction for lithium ion battery applications.

It would additionally be possible to form a porous template or siliconnanowire arrays out of a specific type of silicon (say n-type) andutilize an alternate technique like the vapor, liquid, solid (VLS)process to fill the pores with p-type silicon nanowires resulting in anovel n/p junction configuration which could be utilized in a widevariety of optoelectronic (LED, photovoltaic) and electronic(transistor) applications. (For some general information about the

VLS process see reference (p).) This process is especially favorablesince the Ag particles that catalyze the etching of the siliconsubstrate to form the template could also be used to catalyze the wiregrowth (e.g., in VLS or VSS) at the base of the pore to synthesize thewire. In addition, the Ag particles could serve as electrical contactsfor the device. A wide variety of materials other than silicon can beformed inside the template as well. A few examples are Bi, Ge, GaN, ZnO,and GaAs.

Processes of the invention may be used to create nanostructures whichmake silicon into an intermediate band photovoltaic material (IBPV).(See reference (n).) Silicon has an excellent band structure for IBPV,provided that the strength of particular electronic transitions can beenhanced. One way to do this is to form a dense array of siliconnanowires with specific control over the wire diameter, doping andcrystallographic orientation, as described in reference (i). Processesof the invention may be used for manufacturing such nanowire arrays.

The following references are of interest in relation to thisapplication: (a) K. Peng, Z. Huang, and J. Zhu, Adv. Mater. 16 (1)(2004) 73-76; (b) T. Qiu, X. L. Wu, X. Yang, G. S. Huang, and Z. Y.Zhang, App. Phys. Lett., 84 (19) (2004) 3867; (c) H. Fang, Y. Wu, J.Zhao, and J. Zhu, Nanotechnology 17 (2006) 3768 and Y. Yang, P. Chu, Z.Wu, S. Pu, T. Hung, K. Huo, G. Qian, W. Zhang, X. Wu, Appl. Surf Sci.254 (2008) 3061 and X. Li and P. Bohn, Appl. Phys. Lett. 77 (16) (2000)2572 and H. Asoh, F. Arai, S. Ono, Electrochem. Comm. 9 (2007) 535; (d)K. Peng, J. Hu, Y. Yan, Y. Wu, H. Fang, Y. Xu, S. Lee, and J. Zhu Adv.Mat. 16 (2006) 387; (e) Z. Huang, H. Fang, J. Zhu, Adv. Fun. Mat. 19(2007) pg. 744; (f) K. Peng, M. Zhang, A. Lu, N. Wong, R. Zhang, S. Lee,App. Phys. Lett. 90 (2007) 163123; (g) U.S. Provisional PatentApplication No. 61/044,573, filed Apr. 14, 2008; (h) U.S. ProvisionalPatent Application No. 61/195,872, filed Oct. 9, 2008; (i) U.S. PatentApplication Publication No. 2007/0278476, filed Feb. 27, 2007; (j) L.Tsakalakos, J. Balch, J. Fronheiser et al. App. Phys. Lett. 91 (23)(2007) 2331176; (k) M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes etal., Nano Lett. 8 (2) (2008) 710-714; (1) C. K. Chan, H. Peng, G. Liu,K. Mcllwrath, X. F. Zhang, R. A. Huggins, and Y. Cui Nature Nanotech. 3(2008) 31-35; (m) U.S. Published Patent Application No. 2007/0190542,filed Oct. 3, 2006; (n) A. Luque, A. Marti, Phys. Rev. Lett. 78 (26)(1997) 5014-5017; (o) Q. Shao, A. A. Balandin, App. Phys. Lett. 91(2007) 163503; (p) Y. Cui et al., App. Phys. Lett. 78 (2001) 2214-2216.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties. However, where apatent, patent application, or publication containing expressdefinitions is incorporated by reference, those express definitionsshould be understood to apply to the incorporated patent, patentapplication, or publication in which they are found, and not to theremainder of the text of this application, in particular the claims ofthis application.

What is claimed is:
 1. A nanostructured array comprising polycrystallinesilicon, wherein the nanostructures of the array have lengthwisedirections disposed at a non-zero angle to a silicon-containingsubstrate, wherein the array is formed by etching a substrate composedprimarily of polycrystalline silicon.